In manufacturing lenses, and particularly lenses for eyeglasses, the use of plastics is often desirable due to their light weight and durability. Plastic lenses also provide relatively economic vision correction. Methods for producing plastic lenses of various prescriptions are well known. Applicant's U.S. Pat. No. 4,873,029 and co-pending applications Ser. Nos. 190,856 (filed May 6, 1988) and 339,217 (filed Apr. 17, 1989), which are incorporated herein by reference as if fully set forth, disclose methods for making plastic lenses of ophthalmic quality for eye glasses.
Prior methods of others have, however, failed to provide fast and economic means for manufacturing high index-quality, reliable multifocal (e.g., bifocal, trifocal, etc.) or progressive plastic lenses. U.S. Pat. No. 3,248,460 (the "'460 patent") discloses means for casting plastic lenses from thermosetting or thermoplastic materials wherein a plastic blank having significantly less curvature than required for the full intended prescription of the lens is used as a base onto which an additional layer of material is cast. The '460 patent employs a conventional optical gasket to provide space between the plastic blank and the mold and to hold the resin material in the cavity created thereby. The additional layer of material changes the curvature of the resulting lens over the vast majority of its surface, thereby changing the prescription of the resulting finished lens to the power required. The material in the '460 patent is cured by heat. However, such heat curing process requires heating over a period of more than 12 hours, thus making the formation of the lens a long, drawn-out process.
U.S. Pat. No. 3,946,982 also discloses methods for casting an entire lens surface with a prescription layer using a conventional optical gasket.
Conventional industrial lens casting techniques require the use of "conventional optical gaskets" which hold together the components used to cast the lens, allow for thickness to be cast into the resultant lens, and create a substantially air-tight environment for the casting process. In most cases these conventional optical gaskets can only be used one time and then are discarded. Therefore, a significant number of different gaskets must be maintained.
In-office lens casting is even more demanding with regard to the number of different conventional optical gaskets needed and the inventory necessary to produce different finished lens prescriptions. In one such system, approximately 737 conventional optical gaskets must be maintained in inventory and constantly replaced (after one use) to allow production of all prescriptions. Approximately 200 "optical center movers" (OCMs) must also be inventoried to relocate or decenter the optical center. These OCMs are also not reusable and must be constantly replaced. The need to maintain and replace this varied inventory of conventional optical gaskets and OCMs contributes significantly to the cost of lens casting. In the case of in-office lens casting these components can account for approximately 32% of the materials cost of casting a lens using such a system.
Others have tried to manufacture multifocal or progressive plastic lens, using a lamination technique. Such a technique joins a preformed plastic section to another cured plastic prescription lens. A portion of the preformed section defining a multifocal or progressive region of the finished lens is joined to the prescription lens by adhesive. Such methods have proved to be technologically cumbersome and uneconomical, however, due to the expense of maintaining a large number of preformed lens portions such that all of the possible permutations of patient primary correction and multifocal correction can be formed. Furthermore, the optical quality of such lenses has been suspect because of the difficulty of matching the surfaces of the preformed lens and the wafer.
In the case of in-office lens casting which casts the finished prescription, and to a lesser extent industrial lens casting which casts primarily semi-finished lens blanks, a prism effect may also need be accommodated in the molding process. Present methods for creating prism in plastic lenses have also proven cumbersome. "Prism" is created in lens designs to shift the optical center of a lens from the geometric center of the lens to some other preferred location. Also in the case of progressive lenses, it is used for a compensating base-down prism to offset the base-up prism produced by the progressive mold. In multifocal lenses it is advantageous to shift the optical center of the distance portion of lens to more closely align with the multifocal region of the lens, thus easing the wearer's transition from the distance prescription to the prescription of the multifocal region (near region) of the lens.
When casting a finished lens, prism is cast into the lens in ways that are well known in the art. However, in cases of semi-finished lenses, in order to create prism the lenses must be surfaced to produce both the desired prismatic effect and the correct optical prescription. Surfacing requires additional equipment and time which make such methods less than desirable for making lenses quickly and inexpensively from start to finish.
When reviewed from an overall perspective of lens production, starting with liquid resin and ending with a finished lens mounted in the frame, the conventional process is extremely complex, long and arduous. Curing has required 12-14 hours; wholesale lab surfacing of the cured lens semi finished blank, approximately an additional 30 minutes; and finishing the lens, another approximately 30 minutes. Thus, the overall lens manufacturing process can require 13-15 hours, making it difficult to quickly provide prescription lenses on request, unless one stocks semi-finished blanks and utilizes surfacing equipment, both of which add tremendously to the overall cost of production which is ultimately passed on to the consumer.
It would, therefore, be desirable to provide a faster, more economical method and much simpler for producing multifocal or progressive lenses. It would also be desirable to provide a method for changing the prescription or lens design (i.e., multifocal, progressive, prismatic effects, etc.) of a preformed prescription plastic lens which is both fast and inexpensive. Preferably, such method should produce lenses without employing a conventional optical gasket.